FIG. 1 gives a diagram of an OMT called a “linear polarization separator”, which is made according to the microwave-frequency waveguide technology. This OMT, reference number 1, essentially comprises a first port 2 designed to be connected to a horn facing a microwave-frequency telecommunication antenna and two other ports 3, 4 designed to be connected to a transmitter or a receiver. This OMT operates only with linear polarizations. These three ports are coaxial. The port 3 corresponds to the horizontal polarization and the port 4 to the vertical polarization. The port 3 is rectangular and is connected to the port 2 by one or more waveguide segments 5 having dimensions that are mid-way between those of the ports 2 and 3. The port 4 is connected radially to the port 2 by two waveguide segments 6A, 6B placed symmetrically relative to the common axis of the three ports and each having approximately a “U” shape that is elongated and culminating in coupling slots that are diametrically opposed to each of the ports 2 and 3.
The coupler 7 of FIG. 2 is a “pyramid-shaped” OMT. It comprises essentially a central cavity with a parallelepipedal body of square section and a pyramid 8 placed at the bottom of this cavity. Ports 9 to 12 culminate facing the four lateral triangular surfaces of the pyramid of the parallelepipedal body. With such an OMT, the coupling of the electromagnetic waves between the central port with square section and the four ports can be wide-band. This range of operation can be affected or reduced with the use of a transition between the ports of circular section and the parallelepipedal body of the OMT promoting the propagation of the higher-order modes. Moreover, this coupler has no multiplexing function.
FIG. 3 shows a conventional OMT 13 with circular cross sections. It essentially comprises three successive coaxial waveguide segments 14, 15 and 16 that are generally cavities. The first guide 14 has the largest diameter and comprises two or four rectangular coupling slots like the slot 14A, the only one shown in the drawing, each associated with a port like the ports 14B shown in the drawing. Similarly, the segment 15, with a smaller diameter than that of the segment 14, comprises two or four coupling slots 15A each associated with a port 15B. Finally, the segment 16, with a smaller diameter than that of the segment 15, forms the port for propagating the highest frequency band, while the segment 14 couples the lowest frequencies and the segment 15 couples the frequencies of intermediate value. Such a coupler therefore allows multiband coupling, but the widths of these bands are small.
The coupler 17 of FIG. 4 is of the type that comprises a cavity 18 in the form of a rectangular parallelepiped extended by a parallelepipedal cavity with a square or rectangular cross section and a port 19 with a square or rectangular cross section and being coaxial with the axis of the cavity. The cavity 18 comprises, on each of its two (or four) lateral faces, a coupling slot 18A associated with a coupling port 18B. Such a coupler operates for a relatively wide frequency band, but the transition (not shown), serving as an interface to the connection of a horn of circular cross section, and situated between the cavity 18 with a square or rectangular cross section and the waveguides of circular cross section that are connected thereto, reduces its operating range because of the presence of higher-order modes, and notably of harmonics, interfering with the propagation of the payload signals.
FIG. 5 shows a diagram of an OMT 20 as known according to the U.S. Pat. No. 6,566,976. This OMT comprises a conical body 21 connecting a port 22 of circular cross section to a port 23 also of circular cross section and having a smaller diameter than that of the port 22. Coupling slots 21A associated with ports 21B are made on the conical body 21. Such an OMT makes it possible to propagate only narrow frequency bands.